A solid polymer membrane-type fuel cell is, for example, provided with a stack constituted by laminating a plurality of cells in which a solid polymer electrolyte membrane is sandwiched between an anode and a cathode. Hydrogen is supplied to the anode as a fuel, and air is supplied to the cathode as an oxidizing agent, with hydrogen ions generated by the catalytic reaction in the anode passing through the solid polymer electrolyte membrane to the cathode, where an electrochemical reaction with oxygen occurs, resulting in the generation of electricity.
Moreover, in fuel cells such as a solid polymer membrane type fuel cell, although unreacted air, or off-gas, discharged from the cathode is generally exhausted to outside the system, it is necessary when doing so to check that hydrogen gas is not present in the off-gas.
As seen in Japanese Examined Patent Application Publication No. H06-52662 (hereinafter referred to as Patent Document 1) and Japanese Unexamined Patent Application Publication No. H06-223850 (hereinafter referred to as Patent Document 2), there has been developed a system in which a hydrogen detector is installed in the exhaust system of the cathode side of a fuel cell to check that hydrogen gas does not exist in off-gas.
It is conceivable for such a hydrogen detector to employ a gas contact combustion-type gas sensor, which comprises a detecting element to which a catalyst is bonded, and a temperature compensation element to which a catalyst is not bonded. Utilizing the heat of combustion when the gas to be detected (hydrogen in the case of a hydrogen detector) contacts the catalyst, the gas concentration of said gas to be detected is detected from the difference in the electrical resistance between the detecting element and the temperature compensation element.
Since organic silicon packing and hoses having heat resistance are used for the exhaust system of the cathode side, there exist in the measuring atmosphere of the contact combustion-type gas sensor small amounts of gaseous silicon compounds that precipitate and vaporize from the packing or hoses. Such silicon compounds degrade the performance of the catalyst constituting the detecting element (so-called poisoning), leading to a drop in detection sensitivity to flammable gas over time and a loss in measurement accuracy.
In order to cope with such problems, for a semiconductor sensor as seen in Japanese Unexamined Patent Application Publication No. S56-168542 (hereinafter referred to as Patent Document 3), that is, a sensor that detects changes in electrical conductivity by adsorption of a gas to be detected, by housing the sensor in an environment containing dimethylsiloxane, which exerts a poisoning action on the sensor, to age the sensor, variations in characteristics such as sensitivity between sensors are lessened.
However, no such poisoning measures have been implemented for contact combustion-type gas sensors that detect gas concentration from the difference in the electrical resistance between the detecting element and the temperature compensation element utilizing the heat of combustion when the gas to be detected (hydrogen in the case of a hydrogen detector) contacts the catalyst.
In light of these circumstances, it is a first object of the present invention to provide a contact combustion-type gas sensor that can maintain the initial sensitivity over a long period of time in the presence of silicon vapor.
It is a second object of the present invention to provide a contact combustion-type gas sensor that can minimize as much as possible the loss in sensitivity resulting from adhesion of silicon even when installed in an environment where organic silicon compounds are present.
It is a third object of the present invention to provide a manufacturing method for the aforementioned contact combustion-type gas sensor.